It has been found that certain food products cook and taste best when heated at a specific temperature for a set period of time. As a result, restaurants and food service establishments, especially franchise food chains, have instituted strict criteria for preparation of fried food products. Consequently, restaurants and food service establishments will often require a deep fat fryer which can operate and maintain specific heating parameters.
Deep fat fryers are widely used in commercial food vending establishments, such as fast food restaurants, to heat food products, such as potatoes, fish, chicken, or the like. Accordingly, desirable characteristics in a deep fat fryer include rapid heating, without overshoot, to an operator selected cooking temperature, accurate maintenance of the cooking medium temperature to a temperature within a narrow range around the operator selected cooking temperature, minimal wearing of heating element components, and safety features which prevent injury to the operator or damage to the fryer.
Prior art fryers known heretofore typically include a vat for holding a cooking medium, temperature selection means for inputting a desired cooking temperature for a food product, a heating element (e.g., a gas burner or electric element) for heating the cooking medium, temperature sensing means for sensing the temperature of the cooking medium, and a fryer controller for providing overall control of fryer operations. One significant function performed by the fryer controller is control of the heating element.
The heating element is operated in a melt mode, a post-melt mode, an idle mode, a cook mode, and a boil mode. In the melt mode, a "cold" cooking medium is heated at a slow rate to gradually introduce heat to the cooking medium. Since many cooking mediums are solid at room temperature, special care must be taken in operating the fryer to melt the cooking medium. When solid cooking mediums are quickly heated, hot spots can develop which may damage the fryer and which may scorch the cooking medium, making it unsuitable for use in cooking. Fire or heavy smoking are also possible results of quick heating of said cooking mediums.
The post-melt mode quickly heats the cooking medium to reach an operator selected setpoint temperature (i.e., cooking temperature) which is optimum for cooking the food product.
The cooking medium is maintained at a temperature around the operator selected setpoint temperature in the idle mode. In this mode of operation, the fryer awaits introduction of food product into the vat.
In the cook mode, food product is introduced into the cooking medium, and depending on the load size, may cause a drastic drop in the temperature of the cooking medium. It is during this mode that the food product is cooked.
In a boil mode, the cooking medium is removed from the vat so that a cleaning operation can take place. In this respect, water and detergent are introduced into the vat and heated to a predetermined temperature (e.g., approximately 195.degree. F.).
Referring now to the melt mode, prior art systems turn the heating element on at constant intervals (i.e., pulse) to gradually introduce heat energy into the cooking medium. Once a predetermined melt-release temperature is reached the melt-mode ends, since the cooking medium may now be quickly heated to the operator selected setpoint temperature without any adverse effects.
With respect to the post-melt mode, the prior art utilizes generally two approaches. In the first approach, the heating element is turned unconditionally on (i.e., full ON), until the temperature of the cooking medium exceeds a predetermined threshold temperature a predetermined number of degrees below the operator selected setpoint temperature. Once the cooking medium has exceeded the threshold temperature, the fryer controller begins pulsing the heating element.
In a second approach to the post-melt mode, the heating element is turned full ON until a predetermined threshold temperature is reached. When the cooking medium reaches this threshold temperature, the heating element is turned off, and the internal heat capacity of the fryer is relied upon to cause the temperature of the cooking medium to continue rising until reaching the operator selected setpoint temperature.
With regard to the idle mode, prior art systems employ several different approaches. A first approach is known as ON/OFF control. The heating element is either on or off, with no middle state. The heating element is ON when the temperature of the cooking medium is below the operator selected setpoint temperature, and OFF when the cooking medium temperature is above the setpoint temperature. A second approach is known as proportional control. The proportioning action occurs within a "proportional band" around the setpoint temperature. Outside this band, the controller functions as an ON/OFF unit, with the heating element either fully ON (below the band) or fully OFF (above the band). However, within the band, the heating element is turned on and off for short intervals, wherein the ratio of ON time to OFF time is varied based upon the difference between the cooking medium temperature and the setpoint temperature. A third approach is known as PID (proportional with integral and derivative control). PID combines proportional control with two additional adjustments, which help compensate to changes in the system. Integral determines how long the cooking medium temperature has been below the setpoint temperature, and derivative determines how fast (i.e., the rate) the cooking medium temperature is changing.
One feature common to many prior art idle mode control strategies is that they attempt to minimize the peak-to-valley excursions of the cooking medium temperature. The peak-to-valley excursion is the range of cooking medium temperatures obtained around the setpoint temperature. The maximum temperature establishes the "peak," while the minimum temperature establishes the "valley." The peak-to-valley excursion of the cooking medium temperature is usually minimized by periodically pulsing the heating element, wherein the pulses have a fixed duty cycle. In this respect, the pulses of heat are intended to add the heat necessary to balance the heat lost to the surrounding environment.
Referring now to the cook mode, the controller of prior art systems keeps the heating element unconditionally on during the entire cook mode when a "full load" has been introduced into the cooking medium. A full load is a load of food product which is at or near the maximum load size for the fryer. Prior art systems operate in this manner because introduction of food product typically causes a drastic drop in the temperature of the cooking medium. However, when several cook modes are initiated successively, there is a build up of stored energy in the fryer. Thus, it is possible to overshoot the operator selected setpoint temperature when an interval of time elapses between a series of cooks and sufficient energy has built up. Furthermore, when a series of cooks are initiated, the bottom temperature (i.e., the minimum temperature of the cooking medium reached after the introduction of food product to the cooking medium) will rise with each successive cook. This also occurs due to heat build-up. Thus, each successive cook mode operation will not be uniform. As noted above, prior art systems operate unconditionally ON throughout each "full load" cook mode, and consequently do not dissipate any excess heat.
With regard to the boil mode, which is provided to carry out a vat cleaning procedure, prior art systems require the operator to manually enter this mode. In this respect, prior art controllers do not sense when water has been substituted for the cooking medium in the vat.
There are drawbacks to the operation of the prior art systems in each mode of operation. With respect to the melt mode, the prior art generally operates the same irrespective of the type of cooking medium. However, it would be advantageous to use different rates of heating depending on the type of cooking medium being used. In this respect, liquid shortening can accept heat at a faster rate, without any adverse affects, than can solid shortening. The prior art fails to provide a controller which can heat the cooking medium more rapidly or bypass the melt mode altogether and begin the post-melt mode at once, when the cooking medium can accept heat at a faster rate. This approach would allow for quicker initial heating of the cooking medium. However, it is also noted that in the case of solid shortening, an unsafe condition can result from bypassing the melt mode. Possible results of rapid heating include damage to the quality of the shortening itself, heavy smoking or fire. Accordingly, the prior art also fails to provide a controller which can recognize the type of cooking medium in the vat in order to avoid unsafe conditions.
There are also disadvantages to the prior art post-melt mode, wherein the heating element is continuously on followed by pulses of heat until it reaches a predetermined threshold temperature below the operator selected setpoint temperature. In this respect, pulsing might not be needed or desired depending on the operating conditions and system parameters. For example, if the temperature of the shortening is close enough to the setpoint temperature when the continuous heating is terminated, then the internal heat capacity of the fryer may be capable of raising the cooking medium temperature to the setpoint temperature. This phenomenon is commonly referred to as "thermal lag," and can cause the temperature of the cooking medium to arrive at the setpoint temperature without the further application of heat. Furthermore, in some cases, the pulses of heat may not be sufficient to raise the temperature of the cooking medium to the setpoint temperature. This problem may arise because the duration of each heat pulse is not long enough to overcome heat loss to the surrounding environment. Accordingly, the prior art does not have the ability to adapt to post-melt mode conditions which may differ each time the post-melt mode occurs.
The alternative prior art approach to the post-melt mode, wherein continuous heating is followed by a heat cutoff, has similar drawbacks. In this respect, the prior art does not provide for an adjustable heat cutoff temperature. The heat cutoff temperature should vary, since the resulting peak temperature obtained after the heat cutoff cannot be assured each time the post-melt mode occurs. In this respect, the prior art does not adjust the cutoff temperature for different post-melt mode conditions which may be present.
In general, the prior art approaches fail to provide a controller having a post-melt mode wherein the threshold temperature is modifiable for a subsequent system startup, based upon the peak temperature reached following the heat cutoff during the proceeding system startup. Furthermore, prior art systems fail to provide a controller which adjusts the threshold temperature based upon the rate of rise of the cooking medium temperature during the post-melt mode.
The idle mode of prior art systems also has several drawbacks. In this regard, different system and operating conditions may require more or fewer pulses of heat, consequently frequent control of the heating element may be required to maintain the operator selected setpoint temperature during the idle mode. The very nature of the prior art approach to the idle mode results in many operations of heating element components, thus reducing the life of these components. In many cases, tight control of the cooking medium temperature is not as beneficial to the cooking process as is the extension of the life of the components comprising the heating element. The prior art fails to provide a controller that allows the operator to select an acceptable band for the peak-to-valley temperature excursion, so as to maximize the life of heating element components.
There are disadvantages to the cook mode of prior art systems as well. In this respect, prior art cook modes fail to compensate for the build-up of stored energy, which occurs when successive cooking operations are initiated. Accordingly, at the end of a series of "full load" cooks the cooking medium temperature can overshoot the operator selected setpoint temperature by an unacceptable amount due to the build-up of stored energy in the system. In addition, each cook in a series of cooks will have a different "bottom temperature" as a result of heat build-up. Therefore, each cook in the series will not be uniform.
The prior art's manual procedure for entering the boil mode poses a safety hazard. In this respect, if the vat is filled with water and the controller believes the system is preparing to cook (i.e., begins a start-up cycle), too much heat will be applied to the water, and a boil-over condition could occur. In this respect, damage to the cooking appliance could occur and anyone in close proximity could be injured.
A second aspect of the present invention relates to time compensation, during the cook mode. Time compensation is necessary for convenient operation of the fryer, since the time for the temperature of a food product itself to reach a predetermined "fully cooked" temperature will vary based upon the quantity of food product in the vat and the temperature of the cooking medium during the cook mode. In this respect, it would be advantageous to provide time compensation so that an operator can enter the same cook time each time the same type of food product is being cooked, without concern for the quantity (i.e., load size) of food product introduced into the vat and variations in cooking medium temperature during a cook mode operation.
The present invention addresses the foregoing and other problems, and is directed to an electronic control system and more specifically to an electronic control system having a programmable microcontroller and associated peripherals, for use with heating apparatus, such as fryers, ovens, pressure cookers, pasta cookers, holding cabinets, furnaces, and water heaters.